Fracture coalescence, which plays an important role in the behavior of brittle materials, is investigated by loading pre-fractured specimens of gypsum?, used as a rock model material, in uniaxial and biaxial compression. Several new phenomena and their dependence or? geometry and other conditions are observed. The specimens have two pre-existing fractures or flaws that are arranged in different geometries, and that carl be either open ol closed. Two different test series are performed with these flaw geometries, one under uniaxial loading and one with biaxial loading in which confining stresses of 2.5, 5.0, 7.5 and 10 MPa are applied. As the vertical (axial) load is increased, new cracks emanate from the flaws and eventually coalesce. Flaw slippage, wing crack initiation, secondary: crack initiation, crack coalescence, and failure are observed. Two types of cracks occur: wing cracks, which are tensile cracks, and secondary cracks which initiate as shear cracks in a plane roughly co-planar with the flaw. The secondary cracks usually propagate as shear cracks in the same plane hut, depending on the geometry, they also propagate out of plane as either tensile ol shear cracks. The wing clacks initiate at the flaw tips for uniaxial or low confinement biaxial conditions bur move to the middle of the flaw and disappear completely for higher confining stresses. Thr ec types of coalescence, which depend on the geometry of the flaws and to some extent on stress conditions, occur, they, car? be distinguished by different combinations of wing cracks and secondary cracks. For closed flaw specimens, at lens; partial debonding and slippage of the flaws is required prior to initiation of a crack. In uniaxial compression coalescence and failure occur simultaneously,, while failure in biaxial compression occurs after coalescence. (C) 1998. Published by Elsevier Science Ltd. All rights reserved.
